With the ever increasing demand for energy coupled with the depletion of traditional non-renewable energy sources (i.e., fossil fuels), there is a strong need for development of alternate fuels, particularly “renewable fuels.” Thus, there is much interest in the production of biofuel. Biofuels are viable alternatives to fossil fuels (e.g., petroleum, natural gas) because they comprise a wide range of liquid, solid biomass, or biogas fuels that are in some way derived from a carbon source that can be rapidly replenished (including for example hydrocarbons derived from or produced by biological organisms).
During the production of biofuel, a large quantity of water and alcohols are produced requiring adequate and cost-effective separation. For example, one of the leading biofuel production methods is enzymatic hydrolysis and fermentation, which generates alcohols, such as ethanol and isobutanol at lower concentrations (e.g., ˜0.5-10 wt. %) in water. In addition to the desirability of recovering the low concentration alcohols for use in biofuels, it is also important to separate the lower concentration alcohols from water during the course of biofuel production because the enzymes utilized can lose activity in the presence of alcohols at a concentration of ˜0.5-10 wt. %. However, conventional separation methods, such as distillation, are not economical due to the large amount of water present. Furthermore, simultaneous separation of the alcohol product from the reactor is desirable because the process can proceed in a substantially continuous manner thereby reducing downtime and improving productivity.
Thus, various porous materials (e.g., activated carbon, zeolites) have been investigated for separation of alcohol-water mixtures. One class of compounds of interest is metal-organic frameworks (MOFs), which comprise various inorganic clusters and organic linkers resulting in a wide range of surface areas and porosities. A subfamily of MOFs is zeolitic imidazolate frameworks (ZIFs), which have zeolite-like topology and are constructed by using tetrahedral transition metal ions and imidazolate-based bridge ligands. While most studies utilizing ZIF materials have focused on gas adsorption and separation, some ZIF materials have been studied for separation of water-alcohol mixtures, such as in membrane separation. For example, Liu, X.-L. et al. studied membranes containing ZIF-8 nanoparticles and ZIF-7 nanoparticles for pervaporation recovery of isobutanol from aqueous solutions and found that the ZIF-8 membrane had a higher isobutanol permeance compared to the ZIF-7 membrane (“isotherms of ZIF-7 nanoparticles show insignificant adsorption of isobutanol”). Angew. Chem. Int. Ed., 50: 10636-10639, 10638 (2011). Dong, X. et al. report alcohol (methanol and ethanol)-water separation with a ZIF-71 membrane. Chem. Commun., 49: 1196-1198 (2013). Additionally, U.S. Patent Publication No. 2014/0212940 reports ZIF-containing (e.g., ZIF-4, ZIF-5, ZIF-7, ZIF-8) membranes and processes for removing alcohols from water.
Nonetheless, as noted by Dong, X. et al., within the field of liquid mixture separation, there are formidable challenges associated with screening of suitable ZIF materials due to, for example, the many properties associated with ZIFs, such as pore size, structure, surface chemistry, and thermal and chemical stability. Furthermore, it remains difficult for current adsorbent materials to selectively adsorb trace amounts of organic compounds, such as alcohols, from liquid mixtures. Therefore, there is a need to provide additional adsorbent materials with both improved adsorption capacity and selectivity for organic compounds, such as methanol, ethanol, propanol and butanol, in water, which can be used in separation processes during the production of biofuel.